This invention relates to the manufacture of closures such as caps and the like, and more particularly, to a method and apparatus for injection molding plastic closures.
The injection molding of small plastic articles of even relatively simple designs often utilizes expensive and complex molding machines as exemplified by molding systems used for small plastic articles such as threaded closures for plastic, glass or other containers and bottles. A typical injection molding machine employs forming elements which are subjected to elevated temperatures and pressures. The molding machine components subjected to such conditions must be extremely rugged, necessitating the utilization of durable and expensive materials which require appropriate maintenance, repair and replacement on a regular schedule.
In one known system for the injection molding of small plastic closures, an inner mold in combination with an outer mold defines a forming volume or cavity in which a thermoplastic resin such as polypropylene or the like is injected under pressure at an elevated temperature. The various mold components must be moved relative to one another to extract or eject the molded closure from the cavity.
The art of injection molding plastic closures is well known. Depending upon the design of the plastic closure, including the thread type or other mechanism for engaging the container, the closure may be ejected from the mold in a variety of ways. If the plastic material being molded is flexible or resilient, the closure may be stripped by a commonly known stripper ring pushing the article off of a fixed inner mold core. The molded part must be sufficiently solidified so as not to fold over onto itself during ejection but sufficiently elastic to return to essentially its original molded shape after the threads or other internal projections have been stretched over the core. If the molded material does not possess the appropriate characteristics of flexibility and rigidity for this method of ejection, the article will be damaged or may not return to its original shape and size. Moreover, a very defined or deep thread profile is inherently prone to stripping damage. Also, the closure often has other delicate or fragile features such as a tamper evident ring, which could be damaged even if an otherwise acceptable plastic is being injection molded.
Plastic closures can be manufactured with different features, such as continuous threads, partial or interrupted threads, or spaced hold-down lugs used commonly in child resistant safety bottles. Additionally, a closure may include a retaining rim which is used to secure a disk or insert adjacent the under surface of the upper panel of the closure. Threads, lugs or other projections from the skirt portion of the closure commonly include an undercut surface which is known to provide a more secure engagement with the threads or other mechanism on the container to which the closure is applied.
Although specialized molding systems have been proposed for the manufacture of closure caps having interrupted threads or lugs, particularly those with an undercut, such systems often include very complicated molding schemes. Commonly, a system for the manufacture of closures having interrupted threads, lugs or the like with an undercut surface includes what is usually referred to as a "collapsible" core. The collapsible core includes axially extending segments held together in an assembled configuration to form a portion of the inner mold. After the thermoplastic resin is injected into the cavity in the mold, these segments are advanced forwardly and collapsed radially inwardly towards the axis of the mold to permit each segment to clear the threads or lugs and permit ejection of the closure without interference. Subsequently, the segments are repositioned to form the inner mold and the cycle is repeated.
However, known collapsible cores of the type described are very expensive and the construction of such molds inherently limits the industrial applicability and reliability of such molds. The complicated and often intricate interaction between the various segments and components of the injection molding apparatus presents significant maintenance and reliability problems. The timing and precise interaction between the various components is critical for the proper operation of known systems. Moreover, the frictional interaction between the various moving components of an injection mold having a collapsible core presents additional reliability and maintenance problems.
Further complicating the industrial applicability of known injection molding systems is the difficulty of using such systems with stack molds in which a plurality of molding systems are mounted in adjacent plates. As the plates are shifted relative to one another, multiple closures are formed and ejected providing for the manufacture of thousands of closures from a single machine in a very short time frame. However, failure of one mold component necessitates service to the entire machine thereby sacrificing a significant amount of production capability while service is being performed.
Another problem with known injection molding systems of the type described is the difficulty in reconfiguring the molds for the production of a variety of different closure designs. Specifically, tamper evident closures are widely used to demonstrate to the final consumer that the contents of a container have not been contaminated subsequent to the time the cap was initially secured to the container. One type of tamper evident closure employs a band connected to a bottom edge of a skirt portion of the closure by a plurality of axially extending discrete, small frangible bridges or other members that are circumferentially spaced around the closure. The band includes an inside annular rib which, in use on the container, is located below a cooperating outwardly extending rib on the neck of the container. As the closure is twisted off of the container, contact between the outside rib on the container neck and the inside rib on the band breaks the previously mentioned frangible bridges, thereby separating the band from the remainder of the closure.
Providing a molded closure with a tamper evident band as described complicates the manufacture of the closure in several respects. The formation of the band and the frangible bridges connecting the band to the skirt of the closure requires that the mold used to form the closure have corresponding recesses and protrusions which complicate the removal of the closure from the mold. Naturally, the closure must be removed from the mold in a way that does not break the frangible bridges of the closure since, of course, otherwise the closure cannot be used in the intended manner. The capability for an industrial injection molding machine to satisfactorily mold a closure having such a tamper evident band to date has been suspect. Furthermore, the ability to re-configure a given molding machine for the production of closures without tamper evident bands to/from the production of closures with tamper evident bands often requires significant down time thereby minimizing production capacity and efficiency.